Märklin Digital for Dummies ^® Occupied Track Feedback

When adding a computer with software for monitoring and control of your layout, you also have an option to bring eyes to your computer; Why eyes? Well, if you want to enable automatic operation of trains, the computer needs a reliable overview of where the trains are, which tracks are free for entrance etc. I.e, the ability to monitor Occupied Track Feedback. In Märklin Motorola standard, this is based on the s88 module.

As a start, you need a contact on the Märklin M track. There are two types of contacts:

Transient contacts are switch tracks (activated by the loco pick-up), Reed contacts (activated by a magnet attached under the rolling stock/locomotives) and infrared activation. According to several experts, all the above are not very suitable for computer control as they only issue a short pulse. To receive a feedback, two emitters/transmitters and a relay are necessary at each feedback point to achieve a longer feedback. This is not visible on a model railroad layout with computer control, as quite a number of feedback points are required.

Computer control requires permanent contacts, known as contact track (not to be confused with switch track). A piece, a "track", with both outer tracks electrically insulated from each other and connected to the input of a s88. As a locomotive/carriage passes with its metal wheels/axles over the contact track, they connect the outer rail to ground. Contact tracks are closed (making contact) as long as a train passes over it or stops on it. Therefore, these contact tracks are very suitable and can easily be achieved in the Märklin H0 system. When using K-track by Märklin, cut one rail to the desired length (e.g. with a Dremel tool) and connect it to the s88. It is as simple with C-track by Märklin: Disconnect the ground connector on one side of the track connectors; Example:

1. A track has two rails, the left and the right rail.
2. You decide to cut the ground from the left rail.
3. Then you have to cut the ground from the both ends from the left rail.
4. The left rail should be connected to the s88 module.
5. When the train passes the track, the electric pulse is transmitted from the right to the left rail via the train wheels (as the Märklin wheels are not isolated) to the isolated rail.
6. The electric pulse is transmitted to the s88 module, as long as we have a train on the isolated rail.

Using Märklin M track, you have the following options:

Buying Märklin contact tracks for occupied track feedback will get VERY expensive, as an example, the 90 mm Märklin 5145 contact track costs more than 40 SEK / 4 USD each, and imagine if you want several longer track areas. In addition, using the above methods (i.e. also including bullets 1-6 above), with long feedback tracks, the power supply is less reliable (as only one rail is used for the grounding). The preferred option here is to implement short contact tracks, which will provide more stable power distribution.

Homemade contact tracks is an option that goes well for the real craftsman, but take a look at the Homemade Contact Tracks site, and you can probably understand why this digital dummy wasn't too excited. Also, this alternative provides too short contact tracks for reliable occupied track feedback.

I started to look for an alternative solution, and I found it.

The firm Littfinski-Datentechnik (LDT), offer a special occupied track feedback module for Märklin M tracks, as an alternative for the Märklin 6083 s88 decoder. The RM-GB-8 module provides a current detection solution, i.e. the unit has sensors that recognize every powerconsuming vehicle on the monitored section of the track. Instead of connecting a s88 module to one of the outer rails for feedback, the LDT s88 compatible module "RM-GB-8" connects to the inner rail (according to the image below).

This implies that you easily can create your permanent contacts by isolating track sections e.g. with a piece of paper. The contact track sections can be long or short, according to your requirements. Also, you can include turnouts and the wonderful metal Märklin bridges in your isolated tracks, for occupied track feedback. This can't be done with the combination of M tracks and the Märklin 6088 s88 module. 

The 16 inputs of the Märklin s88 are very sensitive on electrical interference. Therefore no electrical wire (e.g. digital current) should cross the cables, which are connecting the inputs of the feedback modules. Especially the simple feedback of occupied tracks by contact tracks can lead to problems as the isolated rail and the connection cable to the feedback module can easily catch electrical interference. This can be avoided by using e.g. feedback module RM-GB-8 which is insensible against electrical interference due to opto insulated inputs.

You will find the complete picture on how to power the RM-GB-8 in the Power Supply section. The RM-GB-8 will work even if you feed electric locomotives with digital power from the catenary (i.e., power line in the air). You need to define parallel isolated contact areas both at the track and at the catenary, and feed both contacts from the same decoder contact. However, the best solution is probably to feed all trains with digital power from the track instead, and use the catenary for visual purposes only.

A common strategy is often to feed the analogue locomotives with analogue power from the catenary, and use the tracks for digital power, to enable use of non-digital equipment on the layout. Now, the RM-GB-8 will not work, as there will be no occupied track feedback communicated.

Another backside with the RM-GB-8, is that you derive only 8 connections per RM-GB-8 (about 60 USD / 600 SEK each) compared to 16 connections per Märklin 6088 (about 85 USD / 850 SEK each).

As an alternative to the LDT module, Viessmann has a product 5233 that, as I consider, has the similar functionality as the RM-GB-8. It is also less expensive, but some people say it isn't as reliable as the LDT unit; They consider the connectors of the Viessmann unit to be "simple and unsafe". Also, the Uhlenbrock module 63340 - described e.g. on the RJFTrains site - has the same features as the LDT RM-GB-8 and Viessmann 5233 current detection modules. The only significant difference is that you need an Intellibox Control Unit for the Uhlenbrock module, as you receive the LocoNet feedback bus instead of the Märklin s88 bus (i.e., you can not use the Uhlenbrock 63340 in combination with the Märklin 6021 Control Unit).

I believe the current detection solution is the best option for all track systems, including both 3-rail Märklin systems and 2-rail DC systems.

Note that the address for the s88 module is set by how they connected, and not e.g. with dip switches. The s88's are connected to each other in serial order, and the PC software will recognize them e.g. as "Address 1", "Address 2" etc., sometimes they are named "Modules". (OK, there is special equipment on the market that enables you to connect the modules as a "tree" instead which is very applicable for larger layouts. Make sure that your software can manage the selected brand of such devices.) For each s88 you will define "Input 1", "Input 2" etc, sometimes "Connections". However, as a standard the s88 provides 16 track connections, and the RM-GB-8 module provides only 8 track connections. This implies that you have to define the RM-GB-8 modules according to the following example:

This section is quoted from LDT:

"As soon as there is no electrical power on the tracks, all tracks are detected as free. Some available products on the market use auxiliary voltage to solve this problem, but these sometimes causing disturbances within the locomotive decoder and therefore do not supply a suitable solution. The feedback module RM-GB-8 has a build-in intelligence (microprocessor), which supplies a solution by integrated monitoring of voltage. In case of loosed power or short circuit there is no inaccurate detection and notification of a "free track" reported back via the feedback bus to the digital control unit or the PC. All conditions on the tracks are "frozen" during this phase. As soon as there is current on the tracks again the actual situations on the tracks will be detected and reported back via the feedback bus."

Fantastic, huh?

After you have performed some real tests etc. to learn more about occupied feedback tracks - and before heading on with the wiring - I recommend that you sit down with a piece of paper and plan:

When using a computer for the control of your layout, you probably define your layout into blocks in which you can control:

In general, the available area for a train at a meeting point, railway station etc., on my layout comprises 6 x 18 cm track = 108 cm. If having more space for the layout, this available area is of course something I would like to extend, to enable longer trains.

Several experts in this subject recommend a contact track section length of minimum 40-60 cm. In general, I use 1.5 x 18 cm track = 27 cm for contact tracks intended for commanding the stopping of a train, i.e. at the each end of the meeting point track. This optimizes the train length capacity on the layout. I always use an additional third contact track at 3 x 18 cm track = 54 cm between these two contact tracks, for occupied track feedback purposes only. This setup gives me a high level of flexibility when designing the blocks - defining where to start, brake and stop the trains - using the Software.

You must also consider that when using the RM-GB-8 for occupied track feedback, feedback is sent from the loco only, and not from the complete train (if not having pickups in the cars for car light purposes). Remember that the locomotive pickup often is near the back of the loco, and that it takes some time for the computer to read the occupied track feedback when a locomotive enters the contact track section.

The longest contact track section is about 150 cm. I always separate horizontal tracks and ramp tracks in different contact track sections to enable automated decrease and increase of speed at the hills. This is not needed for some locomotive decoders that can manage this by themselves. Also, I plan the contact track sections in order to avoid trains to stop uphill (for performance purposes, when the train starts) and in order to avoid trains to stop halfway into tunnels (for visual purposes).

Most tracks - even the turnouts - are implemented in contact track sections on the layout. In total, there are 32 contact tracks sections. The only area of the layout that does not return occupied track feedback is the main switchyard - where the trains always are operated manually - which makes a total of 33 points where power is feeded to the layout. In average, there are more than one feeding point per track meter, which ensures reliable power for the trains.

In the drawing, the contact track sections are defined with aliases (first row) and addresses (second row). Aliases are used when defining logics for software etc., and are defined according to the following abbreviations:

I think it is important to spend some time on finding a logical system that works. Streets in cities like New York are defined in "Avenues" and "Streets". Streets can also be structured, e.g, "West 42:nd Street" etc. This is where I got my inspiration.

The RM-GB-8 occupied track feedback modules (s88) are defined as 1a, 1b, 2a and 2b (according to the above discussion of 8 vs. 16 contacts by module). This implies that the address for each contact track is defined e.g. as: